1. Field of the Invention
The invention generally relates to a lens and an imaging system, and more particularly, to a fixed-focus lens and an imaging system having the fixed-focus lens.
2. Description of Related Art
With the development of display technology, new displays such as a liquid crystal display (LCD), a plasma display panel (PDP), and a projection apparatus have taken the place of the conventional cathode ray tub (CRT). The LCD, PDP, and other panel display may occupy larger domestic market as a result of the smaller thickness. However, the projection apparatus may provide a larger size image (more than 52 inches, for example) at a lower cost, so the projection apparatus may occupy certain market. In addition, the large size image may be seen by more people, so the projection apparatus is convenient for processing a meeting, a displaying bulletin, and showing teaching information. The advantage of the projection apparatus is one of the reasons that the projection apparatus may not be substituted. In recent years, the projection apparatus gradually becomes one of the indispensable home appliances using in home theatre.
In the projection apparatus, the image showing on the light valve is small and fine, wherein the light valve is a liquid-crystal-on silicon panel (LCOS panel) or a digital micro-mirror device (DMD) for example. To project the small image on the light valve onto the screen to form a large image, a projection lens is capable of projecting the image beam from the light valve onto the screen. In addition, in recent years, to project a large image within short distance, free form surface technology is developed. The free form surface is any curved surface expressed through mathematical expressions, and may be nonaxisymmetrical curved surface for example.
For example, the projector HCP-8A developed by Hitachi company adopts a free form lens and a free form reflective mirror, wherein the free form reflective mirror may be used for trapezoidal correction to reduce distortion. In addition, the aspherical lens is used to correct comatic aberration and point spherical aberration. Also, EP patent publication number No. 1965254A1 discloses a projection lens adopting a free form reflective mirror and a free form lens. Moreover, U.S. Pat. No. 6,631,994 discloses a rear projection apparatus adopting the curved reflective mirror.
The above-mentioned projection lenses all project the light valve to the plane screen, and the imaging surface is a plane. However, the projection systems adopting the plane screen may lack in-situ feel when the projection systems adopting the plane screen is used to a video conference. Therefore, some conventional projection systems used in the video conference adopt a curved screen to increase in-situ feel. However, the conventional projection systems adopt a conventional lens to project the light valve onto the curved screen, the imaging surface formed by the conventional lens is a plane, and the imaging surface may not coincide with the curved screen.
To solve the problem that the imaging surface may not coincide with the curved surface, some conventional technology adopts the projection lens with longer focus, but the technology may stretch the projection distance and make the projection system become too large. In addition, because the imaging surface may not coincide with the curved screen, the image on the screen may generate geometric distortion. A conventional improving method is to correct the geometric distortion by adopting the method of a signal process. Although, the distortion image on the screen may be corrected to a normal image after the signal process, the effective pixels have been reduced, and the illumination of the image may be reduced too.